Ceramic materials are excellent in mechanical properties, especially high temperature strength, hardness, thermal shock resistance, as well as oxidation resistance, corrosion resistance, etc., but it is said that they are remarkably inferior to metallic materials in workability.
Recently, high strength and high hardness materials such as cemented carbide materials have come to be shaped by electrical discharge machining, and the technique is making a rapid progress. Electrical discharge machining generally includes wire cutting electrical discharge machining and die-milling electrical discharge machining.
Wire cutting electrical discharge machining is a shaping process which comprises using a thin wire of copper or brass having a diameter of 0.05-0.25 mm, which is wound up under tension, as an electrode, generating discharge energy between the wire and a work piece, advancing the wire like a jigsaw and thus forming a two-dimensional shape.
Die-milling electrical discharge machining is a method in which an electrode of a specific shape is used and the inverted shape is formed by electric discharge between the electrode and a work piece.
Application of electrical discharge machining to ceramic materials, which are believed to be difficult to shape thereby, is lately being studied. As a result, it has been revealed that ZrO.sub.2 and Al.sub.2 O.sub.3 are made shapable by electrical discharge machining by addition of a suitable amount of TiN, TiC, TiCN, TiB.sub.2, ZrN, NbC, etc.; and Si.sub.3 N.sub.4 are made shapable by addition of TiN, ZrN, etc.
In electrical discharge machining, the influence of the working area effect is great. Therefore, it is necessary that minute protrusions exist dispersed and distributed all over the working area in order to easily induce electric discharge and stably conduct the electrical discharge machining. In this case, the smaller the working area, the smaller the degree of distribution, and thus electric discharge tends to occur at localized positions or points. Therefore, it is required to work with a smaller impact coefficient by reducing working speed, discharge voltage, etc. The "working area effect" herein referred to means the drop of working speed when the working area is excessively small.
In the wire cutting electrical discharge machining, the working surface is a small area facing the thin wire electrode and thus the machining always suffers the influence of the working area effect.
When a ceramic body containing electrically conductive ceramics is worked by wire cutting electrical discharge machining, the minute area facing the wire electrode is the particles of the conductive ceramic per se. Therefore, if the conductive ceramic particles are not uniformly distributed in a sintered ceramic body, stable wire cutting electrical discharge machining is impossible.
As a measure for improving efficiency of wire cutting electrical discharge machining, it is thinkable to increase the amount of conductive ceramics to be added. However has been revealed that increase of conductive ceramics impairs the inherent mechanical properties such as strength, hardness, etc. of sintered ceramic bodies.
When conductive ceramics are added, the conductive ceramics must be uniformly distributed in order to stably carry out the wire cutting electrical discharge machining. In this case, if large particle size conductive ceramics are used, the particles cause the working area effect and the wire electrode is broken unless the working speed is reduced.
In die-milling electrical discharge machining, the electrode is remarkably larger in comparison with a wire and the working area is far larger in the working direction than in case of the wire electrode, and the working area effect is effectively exhibited and the areal working speed can be enhanced.
In addition to the above-mentioned workability, sintered ceramic bodies involve the following problems.
Generally in the shaping work using a die, in punching work for instance, work pieces slide on the surface of the die and simultaneously the die per se suffers a great stress. When this is repeated, the die surface becomes abraded. Microscopic observation of the abrasion reveals that it is caused by wearing of particles of the component constituting the tool, drop-off of particles per se of the component constituting the tool and simultaneous occurrence of the two phenomena.
In the case of cemented carbide tools, as they are used repeatedly, work pieces come to be stained, that is, they are contaminated by the cemented carbide material. The cemented carbide tools having suffered surface abrasion can be reshaped by grinding and polishing for further use, although they are discarded when they cannot be reshaped because of their complicated shape.
On the other hand, sintered ceramic tools are free from staining of work pieces and suffer less abrasion because of hardness of the particles and tool life (time period until reshaping is required) is longer than the cemented carbide tools.
Although sintered ceramic tools suffer less abrasion by drop-off of particles because of strong bond between the particles in comparison with cemented carbide tools, once particle drop-off abrasion happens and especially large particles drop off, reshaping is required at this stage. But the tools are discarded if the shape is complicated and reshaping is impossible. Also when large diameter particles drop off, work pieces being shaped by such tools are susceptible to scratching and thus rejectable products are turned out.
The present invention has been completed in order to solve the above-mentioned various problems relating to sintered ceramic tools. That is, the object of this invention is to provide sintered ceramic bodies, from which can be prepared long life metal working tools, which have excellent mechanical properties, hardly cause flaws in work pieces and can be easily manufactured. Further, another object of the present invention is to provide sintered ceramic bodies which are shapable by both wire cutting electrical discharge machining and die-milling electrical discharge machining and can be formed into metal working tools. A still further object of this invention is to provide ceramic tools, which provided with surface smoothness as required in tools which are easily manufactured by electrical discharge machining, have long life as tools, and scarcely cause flaws in work pieces.